Libpng-1.5.10 adds a new function to check the palette-index for valid range. Currently we simply display such pixels as black, which is harmless. The libpng-1.5.0 feature only issues a warning, and we ignore warnings if PR_LOGGING is not defined. For speed, we should avoid this new check which involves another pass over each row of pixels. The embedded libpng will skip the check, but the system library does not. Libpng-1.5.10 provides a "png_set_check_for_invalid_index()" function that can be used to avoid the check.